Ohm's Law Calculator
Use Ohm's Law to solve for voltage (V), current (I), or resistance (R). Enter any two values and the calculator finds the third. Also shows power dissipation (P = VI).
Ohm's Law is the foundational equation of electrical circuits. Discovered by Georg Ohm in 1827, it relates three quantities: voltage (the electrical pressure pushing charge), current (the flow rate of that charge), and resistance (what limits the flow). The relationship is famously simple — V = I × R — and yet it underlies everything from picking a current-limiting resistor for an LED to sizing residential wiring.
This calculator solves for any one of the three variables given the other two, and also computes power dissipation (P = V × I, or equivalently P = I²R, or P = V²/R). Enter the two values you know and choose which variable you're solving for.
A few notes: Ohm's Law holds for "ohmic" materials, where resistance is constant across the operating range. Most metals at modest currents are ohmic. Many semiconductors (LEDs, diodes, transistors) are not ohmic — their resistance changes with voltage — so Ohm's Law alone doesn't describe them. For DC circuits with resistors, though, it's exact.
Inputs
Leave at 0 when solving for voltage
Leave at 0 when solving for current
Leave at 0 when solving for resistance
Results
Voltage
12.00 V
Current
2.0000 A
Power
24.00 W
Ohm's Law Results
| Parameter | Value |
|---|---|
| Voltage (V) | 12.0000 V |
| Current (I) | 2.0000 A |
| Current (mA) | 2000.00 mA |
| Resistance (R) | 6.0000 Ω |
| Power (P) | 24.0000 W |
| Power (mW) | 24000.00 mW |
| Formula Used | V = I × R |
Formula
How to use this calculator
- Choose which variable you're solving for: voltage, current, or resistance.
- Enter the two values you know. Leave the unknown at any value — the calculator ignores it.
- Read the computed value plus the dissipated power. If the power exceeds your component's rating, the part will overheat.
- For circuits with multiple resistors in series, add resistances: R_total = R1 + R2 + ... . For parallel: 1/R_total = 1/R1 + 1/R2 + ... .
- For AC circuits with reactive components (capacitors, inductors), use impedance Z instead of pure resistance R. Ohm's Law then reads V = I × Z for sinusoidal steady state.
Worked examples
LED current-limiting resistor
An LED requires 20 mA at a 2.1 V forward drop. Power supply is 5 V. Resistor must drop: 5 − 2.1 = 2.9 V Current: 0.020 A R = V / I = 2.9 / 0.020 = 145 Ω → use the standard 150 Ω value P = I²R = 0.020² × 150 = 0.06 W → a standard ¼ W resistor is plenty
Household wiring
A space heater draws 12.5 A at 120 V (typical U.S. wall outlet). Apparent resistance: R = V / I = 120 / 12.5 = 9.6 Ω Power: P = V × I = 120 × 12.5 = 1500 W (the heater's rated power) Standard U.S. 15-amp circuits trip at 15 A, so this heater is close to the limit. Two heaters on the same circuit would trip the breaker — which is exactly why circuits are sized this way.
When to use this calculator
Use Ohm's Law for any DC circuit with resistive components — designing or analyzing simple electronic circuits, picking resistor values, sizing wiring for a current draw, choosing fuse/breaker ratings, calculating heat dissipation.
For non-ohmic components (LEDs, diodes, transistors), use the device's I-V curve or specific equations (Shockley diode equation, transistor models). For AC analysis with phase relationships, use complex impedance. For high-frequency RF work, transmission-line effects matter.
A practical tip: when in doubt, calculate the power dissipation. Underpowered resistors are the most common point of failure in hobbyist circuits.
Common mistakes to avoid
- Mixing units. Volts × milliamps = milliwatts, not watts. Use consistent SI base units (volts, amps, ohms) to avoid factor-of-1000 errors.
- Forgetting that LEDs and diodes are not ohmic. You cannot use R = V/I to compute "LED resistance" directly.
- Ignoring power dissipation. A 1 kΩ resistor at 30 V dissipates 0.9 W — far above a ¼ W resistor's rating, so it will overheat.
- Using Ohm's Law on AC circuits with capacitors/inductors without using impedance. For pure resistance the formula is the same, but reactive components introduce phase shifts.
- Confusing voltage drop with absolute voltage. In a series circuit, each resistor has its own voltage drop, and they sum to the source voltage.
Frequently Asked Questions
Sources & further reading
- Ohm's Law — basic electricity — All About Circuits